Technical Abstract:
Understanding mechanisms of xylem transport of calcium (Ca) in green bean plants could provide strategies to increase Ca levels in pods for human consumption. Greenhouse-grown green bean cultivars 'Hystyle' and 'Labrador', which display high and low pod Ca concentrations, respectively, yet similar whole plant Ca influx rates, were used to examine the role of pod transpiration in influencing Ca accretion in pods. Epidermal peels revealed that developing pods of 'Hystyle' had twice the stomatal density of 'Labrador'. However, pod transpiration rates estimated by weight loss of detached pods and measured via a porometer on intact pods were similar for both cultivars under equivalent environmental conditions. The microenvironment surrounding pods of the two cultivars may not be equivalent, however, due to differences in canopy architecture. Relative humidity (RH) sensor data revealed a higher RH environment in 'Labrador' canopies. To test whether pod environment could account for the different pod Ca levels of the cultivars, inflorescences of each cultivar were enclosed and maintained at equivalent RH environments (high or low RH). Ca concentration of mature pods declined by 25% under a high RH environment when compared to unenclosed pods in both cultivars. Under a low RH environment, mature pod Ca concentration increased in 'Hystyle' by 20% when compared to unenclosed pods, but Ca concentration in 'Labrador' pods was unaffected. Because 'Labrador' pod calcium levels did not increase in a low RH environment, transpiration is not the only factor controlling Ca transport to pods. Transported Ca may be sequestered in the stem of 'Labrador' prior to pod delivery. This research was funded by USDA-ARS Cooperative Agreement No. 58-6250-1-003.